Ultra-Sensitive Strain Sensor Based on Flexible Poly(vinylidene fluoride) Piezoelectric Film
- 62 Downloads
A flexible 4 × 4 sensor array with 16 micro-scale capacitive units has been demonstrated based on flexible piezoelectric poly(vinylidene fluoride) (PVDF) film. The piezoelectricity and surface morphology of the PVDF were examined by optical imaging and piezoresponse force microscopy (PFM). The PFM shows phase contrast, indicating clear interface between the PVDF and electrode. The electro-mechanical properties show that the sensor exhibits excellent output response and an ultra-high signal-to-noise ratio. The output voltage and the applied pressure possess linear relationship with a slope of 12 mV/kPa. The hold-and-release output characteristics recover in less than 2.5 μs, demonstrating outstanding electro-mechanical response. Additionally, signal interference between the adjacent arrays has been investigated via theoretical simulation. The results show the interference reduces with decreasing pressure at a rate of 0.028 mV/kPa, highly scalable with electrode size and becoming insignificant for pressure level under 178 kPa.
KeywordsPiezoelectricity PVDF film Tactile pressure Flexible sensor
Piezoresponse force microscopy
Poly(vinylidene fluoride) (PVDF) is a chemically stable piezoelectric polymer material that has many applications in different fields for its pyroelectric, piezoelectric, and ferroelectric properties [1, 2]. Especially, owing to the outstanding mechanical properties (the Young’s modulus 2500 MPa and strength at break point ~ 50 MPa), the pressure sensor based on PVDF shows a good mechanical property such as flexibility and antifatigue [3, 4]. Compared with the commonly used pressure sensors based on ferroelectric PZT family materials, the PVDF-based pressure sensor is nontoxic and biocompatible [5, 6]. Most importantly, the PVDF-based sensor was more soft and tough than PZT-based sensor due to the high flexibility coefficient of PVDF film, which could be made the required shapes for complex strain sensing [7, 8]. Accordingly, the PVDF-based pressure sensor is thought to be one of the potential flexible bio-sensor for pressure characterization in the rapid development of bio-medical field [9, 10]. Sharma et al. designed a pressure sensor for smart catheter with PVDF film; it could be integrated onto a catheter for real-time pressure measurement . Bark et al. developed a pulse wave sensor system to non-intrusively measure heart pulse wave signals from driver’s palms based on PVDF; results show that the sensor system can provide clear pulse wave signals for heart rate variability analysis, which can be used to detect driver’s vigilant state to avoid traffic accidents . Lee et al. fabricated a sensor with PVDF and ZnO nanostructures and it could detect the changes in pressure and temperatures for artificial skin . The sensor, however, only detects pressure at a single point with large dimension.
Real-world applications, such as patched biosensor for detecting the human body pressure, demand multipoint sensing, structurally flexibility, and ultra-high sensitivity [14, 15, 16]. In this reported work, a 4 × 4 flexible sensor array based on piezoelectric PVDF film is demonstrated, showing ultra-high sensitivity of 12 mV/kPa and fast output response of 2.5 μs. The magnitude and spatial distribution of the pressure applied on a human finger are characterized.
Design and Experimental
Design and Fabrication of the Sensor Array
To fabricate the sensor array, a slide glass covered with polydimethylsiloxane (PDMS) was prepared as a stiff substrate. The PVDF thin film covered by Al on both sides was loaded on the substrate. Then, the photoresist was spin-coated on the surface of the film with a speed of 3000 rpm for 40 s. After photolithography and wet etching of Al by a mask aligner system (ABM, Inc., USA), the 16 capacitor units with 4 × 4 square structure were prepared. After that, the flexible sensor on the PDMS substrate was picked up from the slide glass. The electrodes of each capacitor were connected with the conductive wires through silver glue. In order to obtain good bio-compatibility, the sensor was packaged by being covered with PDMS on the top and heated for 12 h at 60 °C. Figure 1b displays a photograph of the bent pressure sensor, illuminating that the sensor is flexible.
Piezoelectric Property of the Sensor Array Based on the PVDF Film
Piezoresponse force microscopy (PFM) study (Seiko, Inc., Japan) was carried out to characterize the surface morphology and piezoelectric properties of the PVDF film of the proposed sensor under an AC bias voltage of 2 V with a scanning area size of 2 × 2 μm2.
Calibration for the Sensor Array
To calibrate the sensor, various pressures were applied on the proposed sensor in an electro-mechanical experimental platform connecting to a data acquisition (DAQ-USB6008) equipment from National Instruments. The data acquisition with four differential analog signals was set with differential model. The output voltage signal from the proposed sensor was obtained by changing the connection between sensor array and the DAQ.
Results and Discussion
In conclusion, a 4 × 4 sensor array with 16 capacitor units based on the piezoelectric PVDF thin film has been fabricated and packaged with PDMS. The sensor array exhibits flexible and high sensitive properties. The hold-and-release output response of the sensor was obtained by applying impulse pressures with various frequencies, which indicated the sensor array could generate 20–300 mV voltage signals within 2 ms when applying a pressure in the range of 60–150 kPa. The obviously different pressure distributions in the finger during the finger movement of human hand have been observed by using the proposed sensor, which is expected to explore the skill of the human fingers more precisely.
The project was supported by research grants from the open fund of the State Key Laboratory of Luminescent Materials and Devices (No. 2018-skllmd-06), the Fundamental Research Funds for the Central Universities (Nos. ZYGX2016J055 and ZYGX2016KYQD131), and the Technology Innovative Research Fund of Sichuan Province of China (Grant No. 2015TD0005). This work was also supported by the National Natural Science Foundation of China (Grant No. 61474016).
Availability of data and materials
The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.
KL and WH designed the experiments and wrote the manuscript. KL and JXG designed and performed the sample fabrication. TXG performed and analyzed the surface morphology of the sensor. XBW and BWL performed and analyzed the piezoelectric results. SYL and BY performed and analyzed the mechanical experiment. All authors contributed to the scientific discussion and edited the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
- 2.Nakamachi E, Uetsuji Y, Kuramae H, Tsuchiya K, Hwang H (2013) Process crystallographic simulation for biocompatible piezoelectric material design and generation. Arch Comput Meth Eng 20(2):155–183Google Scholar
- 4.Meng Y, Yi W (2011) Application of a PVDF-based stress gauge in determining dynamic stress–strain curves of concrete under impact testing. Smart Mater Struct 20(6):065004Google Scholar
- 5.Cauda V, Canavese G, Stassi S (2015) Nanostructured piezoelectric polymers. J Appl Polym Sci 132(13)Google Scholar
- 6.Shu FF (2007) Application of PVDF piezoelectric-film sensor to plantar pressure measurement. In 6th China International Silk Conference/2nd International Textile Forum, pp 322–326Google Scholar
- 13.Lee JS, Shin KY, Cheong OJ, Kim JH, Jang H (2015) Highly sensitive and multifunctional tactile sensor using free-standing ZnO/PVDF thin film with graphene electrodes for pressure and temperature monitoring. Sci Report 5:7887Google Scholar
- 15.Ferrelltorry AT, Glick OJ (1993) The use of therapeutic massage as a nursing intervention to modify anxiety and the perception of cancer pain. Cancer Nurs 16(2):93–101Google Scholar
- 16.Ryu J, Son J, Ahn S, Shin I, Kim Y (2015) Biomechanical analysis of the circular friction hand massage. Technology & Health Care Official Journal of the European Society for Engineering & Medicine 23 Suppl 2(s2):S529Google Scholar
- 17.Shirafuji S, Hosoda K. Detection and prevention of slip using sensors with different properties embedded in elastic artificial skin on the basis of previous experience. In International Conference on Advanced Robotics. 2014Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.